In recent years, active matrix display devices (such as light-emitting display devices and electrophoretic display devices) in which a switching element or a current control element including a thin film transistor (TFT) is provided in each of display pixels arranged in matrix have been actively developed. As one of such light-emitting display devices, for example, an electroluminescent (EL) display device is given.
A technique where a transistor in which a channel formation region is formed in an oxide semiconductor film (hereinafter referred to as an “oxide semiconductor transistor”) is formed over a light-transmitting substrate and used for a switching element or the like of a display device has been studied (see Patent Document 1).
As compared with a transistor in which a channel formation region is formed in an amorphous silicon film, an oxide semiconductor transistor has a high field effect mobility and thus has the advantage of a high on-state current. In addition, the oxide semiconductor transistor has the advantage that the off-state current is lower than that of the transistor in which a channel formation region is formed in an amorphous silicon film.